1. Field of the Invention
The invention relates generally to the field of photographic cameras and in particular to those cameras in which a filmstrip is first prewound from a cartridge onto a take-up spool without exposing any of the frames on the filmstrip and then is rewound one frame at a time back into the cartridge after each exposure is completed.
2. Description of the Prior Art
Typically in a number of 35 mm cameras, there is included a motorized film transport mechanism. In operation, a leading end portion of the filmstrip extending from a light-tight cartridge loaded in the camera is attached automatically to a take-up spool. The take-up spool is rotated after each exposure is completed to advance successive frames of the filmstrip from the cartridge across the focal plane of a taking lens and onto the take-up spool. Each time the take-up spool is rotated, an unexposed frame on the filmstrip is located in the focal plane of the taking lens and an exposed frame is wound onto the take-up spool. When the fresh film supply has been exhausted, a trailing end portion of the filmstrip remains attached to a supply spool inside the cartridge. This end of film condition is signalled by a sudden increase in the film tension and in the motor current as the take-up spool attempts to withdraw the remainder of the filmstrip from the cartridge. A tension sensing mechanism responds to the increase in film tension, or a detecting circuit responds to the increase in motor current, by reversing the motor drive to rotate the supply spool inside the cartridge. The rotated spool draws the exposed filmstrip off the take-up spool and rewinds it into the cartridge. Then, a rear door of the camera is opened and the cartridge is removed in order to process the filmstrip.
If the rear door of the camera is accidentally or inadvertently opened before the filmstrip is completely wound off the take-up spool and back into the cartridge, the exposed frames on the take-up spool may be ruined by stray light. To prevent this occurance, cameras have been recently provided with a motorized film transport mechanism which prewinds substantially the entire length of the filmstrip onto the take-up spool before any exposures are taken. Then, after each exposure is completed, the filmstrip is rewound one frame at a time back into the cartridge. Thus, the exposed frames will be protected within the cartridge from stray light should the rear door be opened prematurely.
Typically, in a camera which first prewinds the filmstrip onto a take-up spool without exposing any of the frames and then rewinds the filmstrip one frame at a time back into the cartridge after each exposure is completed, selection of either a load (prewind) mode or an expose (rewind) mode of operation of the motorized film transport mechanism may be accomplished manually, as disclosed in U.S. Pat. No. 4,251,148, granted Feb. 17, 1981, or automatically, as disclosed in U.S. Pat. Nos. 4,460,256, granted July 17, 1984, 4,504,131, granted Mar. 12, 1985, and 4,586,801, granted May 6, 1986. In each patent, the mode determining means suffers the disadvantage that it is mechanically complex and, therefore, is expensive to implement in a camera. Moreover, there is not provided any positive assurance that film is present on the take-up spool when the expose mode is employed. For example, in the '801 patent, a switch for sensing that a cartridge is present in the camera and that a rear door is closed cooperates with a tension sensing mechanism and a mode change-over member to release the member to effect a change-over from the load mode to the expose mode in response to a sudden increase in the film tension produced by the end of prewinding of the filmstrip onto a take-up spool. However, in the '801 patent, it may be possible for the filmstrip to be tensioned because of a snag before the filmstrip is wound onto the take-up spool, in which instance the expose mode would be introduced prematurely.